Hydraulic transmission



Jly 3, 1945t l. J. SNADER ETAL HYDRULIC TRANSMISSION original Filed June' 9, 1957 3 Sheets-Sheet 1 Y 6Jv o A '25 eww-77!! 92 v20 l. 'x WOXC' July 3, 1945. l. J. SNAVDER Eil-Al. 2,379,546 I HYDRAULIC TRANSMISSION original Filed June 9, 1937 s sheeissheei 2 July 3, 1945. l. J. s'NADER Erm. 2,379,546

HYDRAULIC TRANSMIISSION Original Filed June l9., 1937 3"Sheets-Sheet 3 line 6 6 of Fig. 4.

adjustment.

Patented lJuly 3, 194,5l

f HYDRAULIC TRANSMISSION Ira J. Snader and Max A. Mathys, Detroit, Mich., assignors to Ex'-'Cell0 Corporation, Detroit, Mich., a corporation of Michigan Original application June 9, 1937, Serial No.

147,178, now Patent No. 2,299,234, dated October 20, 1942. Divided and this application July 31, 1940, Serial No.' 348,905

8 Claims.

The present invention relates generally to improvements in hydraulic'transmissions and has `particular reference to a new and improved transmision adapted especially for machine tools.

One of the objects of the invention is to pro vide a novel hydraulic transmission including a motor and a pump for supplying iluid under pressure to the motor in which th delivery of the pump is fully utilized at all times in doing useful work, and is varied as required without the use of relief valves or other accessories commonly employed for by-passing excess iuid.

Another object is to provide a new and improved hydraulic transmission of the foregoing character in which the speed vof the motor, a1- though selectively adjustable, will be substantially constant regardless of varying load conditions and resulting variations in leakage in the pressure side of the system.

A more specific object is to provide a novel hydraulic transmission of the foregoing charembodying the features spirit and scope of the invention as expressed in the appended claims.

The present application is a division of our copending application No. 147,178, illed June 9,

acter in which the delivery of the pump is automatically adjustable in response to a controlv pressure built up in the exhaust line of the motor by an interposed flow restriction.

Further objects and advantages will become apparent as the description proceeds.

In the accompanying drawings,

Figure l is an axial sectional view, taken along line l-,lof Figure 2, of a pump forming part of. a hydraulic transmission embodying the featuresof our invention.

Fig. 2 is an end elevational view of the pump.

Fig. 3 is a fragmentary sectional viewvof thev pump valve taken along line 3-3 of Fig. 1.

Fig. 4 is a fragmentary transverse sectional view .taken substantially along line 4-4 of Fig. 1, and

showing the face of the stationary valve member. Fig. 5' is a fragmentary transverse sectional view taken substantially along line 5-5 of Fig. 1, and showing the face of the movable valve member.

Fig. 6 is a sectional Fig. 7 isa horizontal sectional Aview taken substantially along line 1 1 of Fig. l; but with the detail view taken along volume control valve in a different position of l Fig. 8 is' a longitu/dinal sectional view of a.

modified form of volume control valve. Fig. 9 is a transverse sectional view taken along the line 9-9 of Fig. 8. 5 4:

Fig. 10 is an enlarged fragmentary sectional view of one of the pump pistons.

1937 (Patent No. 2,299,234, issued October 20, 1942).

Brief general rsum of invention The hydraulic transmission (see Fig. 11) constituting the exemplary embodiment of the invention comprises a variable delivery pump l having an intake line 2 and a pressure delivery line 3. The intake line 2 connects with a suitable source of uid supply, such, for example, as a sump or reservoir L The delivery line 3 is'adapted to be connected by a direction valve 5 selectively to one or the other of two lines 6 and 'l leading respectively to opposite sides of a hydraulic motor-,8. This motor may be of the reversible rotary type. Upon connecting the pump delivery line 3 to one of the motor lines 6 and 1, the direction valve 5 serves to connect the other motor line to an exhaust line 9 discharging to the sump 4.

Adjustment of the pump delivery is under the control of an automatic valve l0 responsive to a pressure in the hydraulic circuit reilecting the load conditions of the motor 8. More particularly, the control valve l0 cooperates with adjustable ilow restriction means interposed in the exhaust line 9 to vary the pump delivery in accordance with the motor requirements so that the motor may be operated at different predetermined substantially constant speeds. In the present embodiments, the ow restriction means consists of three parallel adjustable oriilces Il, l2 and I3 selectively available under the control of a valve I4. vThe orifices may be adjusted -to different sizes and serve to build upa low back pressure controlling the speed of the motor 8. Assuming the motor to be connected to drive a reciprocatory machine tool carriage (not shown), the larg- Fig. 11 is a. diagrammatic view of one form 65 est oriiice II when connected is adapted to eect rapid traverse, the smallest orifice |2 when connected is adapted to eiect a slow feed, and the intermediate orifice I3 when connected is adapted to effect a relatively fast feed. In Fig. 11, the control valve I is responsive to the pressure in the line 9 at the upstream side of the orifices Il, I2 and I3, and to this end, is adapted to be connected through a control line I5 by the direction valve 5 when in operative position to the line 9. For any selective orice restriction, tending pressure variations in the line 9 will eiect compensatory adjustments in the pump delivery so that i the pressure dropacross the restriction II, |2 or I3 will remain practically uniform regardless of the motor torque. Consequently, the speed of themotor 8 for any given restriction will remain substantially constant.

The hydraulic pump The particular hydraulic pump I utilized in the transmission comprises a closed housing IS preferably generally rectangular in shape, and having a bottom Wall I1 adapted to be secured to a suitable support (not shown). The housing I6 also has opposite end Walls I8 and I9, and is closed at the top by a removable cover plate 20.

A suitable drive shaft 2| extends longitudinally through the housing I6, and is journaled at opposite ends in the walls I8 and I9. Integral with the wall I9 is a large external cir`cular boss 22 having an axial bore 23 counterbored at the inner end. A stationary valve plug 24, in the form of a single cylindrical block, is tightly pressed into the bore 23, and is located axially by a peripheral end ange 25 seated in the counterbore. To locate the plug 24 angularly, it is formed with a longitudinal groove 216 engaging a pin 21 on the inner end of a screw 28 threaded radially into the boss 22. Formed in the plug 24 is an axial bore 29 which is closed at the outer end,'and which is open and flared at the inner end. A bushing 39 is iitted in the bore 29. and supports one end of the shaft 2|. The opposite end wall I9 is formed with a bore 3| in axial alignment with the bushing 30. Preferably, an antifriction bearing 32 is -ing an intermediate spring space 3S. A plurality of parallel bores 31, six in the present instance, open through the flange 34, and constitute pump cylinders annularly arranged in uniformly spaced relation about the shaft 2|. Similarly, the ange 35 is formed with guide bores 39 in axial alignment with the bores or cylinders 31. Rigidly secured to the outer end face oi the flange 34 is a circular val-:e plate 39 which cio: s the pressure ends of the cylinders 35, and which preferably is formed with circular recesses dening slightly enlarged elosed end extensie-:ifJ of the cylinders.

A plurality of pump plunger.:` or pistons 40 are reciprocably; osed espe/:tively the open ends of the cyliri;` i and are slide th Lr cuter enti. .n the bres f held to a minimum.

The pistons 40 are operable in uniformly timed sequence, with a variable stroke as hereinafter described, by a swash or cam plate 45 which is journaled -on a pin 46 for angular or rocking adjustment into diierent inclined positions. .The pin 46 is pressed in spaced lugs 41 in the housing I6 and is perpendicular to the axis of the shaft 2|. The plate 45 has a iiat annular cam face 48 which freely encircles the shaft and which is engaged by oating shoes 49 operatively connected to the outer ends of the pistons 49.

` The valve plug or stator 24 and the plate 39 have annular end faces 59 and 5I in bearing engagement, and constitute a rotary end face valve for controlling the admission and discharge of fluid respectively to and from, the pump units. Formed in the stator 24 are two arcuate suction and delivery ports 52 and 53 separated by diametrically spaced land areas 54 and 55 and arranged in concentric relation to the shaft 2|. These ports are located at opposite sides of an axial plane intersecting the land areas and perpendicular to the pivotal axis of the swash plate 45. Formed in the valve face 5| of the plate 39 are a plurality of ports 56 which open respectively to the closed ends of the pump cylinders 31,'

and which are annularly arranged in a circle of the same curvature as the ports 52 and 53 for movement successively across the latter upon rotation of the cylinder body 33.

It will be evident that when the swash plate 45 is angularly disposed, rotation of the cylinder body 33 will cause the pistons 40 to be reciprocated in timed sequence and with a simple harmonic motion, and that each piston will bemov-v ing through its suction and pressure strokes while the associated cylinder 31 is in communication respectively 'with the inlet and delivery ports 52 and 53. As the pistons of each set of diametrically opposed pistons reach top and bottom dead centers respectively, the associated cylinders 31 will be closed by the land areas 54 and 55, and at these times `only two cylinders will be open to eachof the valve ports 52 and 53. At all other times, three cylinders 31 will be open to each of the valve ports 52 and 53. Consequently, the pump will discharge continuously, and the rate of discharge will be substantially constant for any given setting of the swash plate 45. If the angular position of the swash plate is changed, the length of the piston strike will be adjusted to adjust the rate and volume of pump delivery. When the plate 45 is adjusted into a plane perpendicular to the shaft 2| no reciprocation will be imparted to the pistons 40.

As an important factor in avoiding noise, the volume of pressure liquid in the closed ends of the cylinders 31 at the end of the pressure stroke is To this end, the piston clearance in the valve plate is made as small as 1s practically convenient. Alffo. the pivot pin sult the inward limit orf-ement of .i e tous 49 -s su'fistantiallff c-ffnsnt and clos;` to the n r nds of the cylinder 31 While the o1 twaid ifm f mevemen if variable n riccordanI wif. cL-.ang 'In the un ul-r 1o-t `Vn of the :atraen pat-4:?.

livery.

the bores 51 are intersected by a recess 58 formed in the periphery of the plug, and registering with y 2,879,5464 bores 51 in the valve plug 2li.4 'I'he outer ends of a passage 59 opening through the peripheral wall of the boss 22. sage 59.

The line 2 is connected to the pas- The delivery port 53 is in communication at the bottom or root with a plurality of Pump adjusting means In its preferred form, the pump adjusting means comprises a control piston 63 acting on the free end of the swash plate 45 in a direction to level oi the latter against the hydraulic reaction in the cylinders 31 and the action of the piston return springs di. lThe control piston 63 is reciprocable in a cylinder 69 opening through the wall I9, and closed at the outer end by a cover or stop plate 65. .The cylinder 64 extends parallel tothe shaft 2i, and preferably is locateddiametrically opposite the pin 46 and outside the circle of revolution of the cylinders 31 so that the thrust of the piston 63 against the plate 45 will have a moment arm considerably longer than that of the aggregate opposing forces resulting from the hydraulic reaction through the pistons 40. The diameter of the piston 63`and the moment arm of the force exerted thereby determine the pressure required in the cylinder 64 in relation to the pump delivery pressure to balance the cam or swash plate 45. By providing a comparatively large diameter and a relatively long moment armas shown, the balancing pressure required in the cylinder 69 will be much lower than the corresponding pump delivery pressure.

The control piston 63 is operatively connected to the swash plate 95 by a pin 66. One end of the pin 66 is formed with a spherical head 61 seated for universal pivotal movementI in a complementa] socket 6B in thefree end of the plate 45, andthe other end is rounded and seated in a complemental socket 69 in the end wall of the piston 63. A coiled compression spring 10 acts to maintain the head 61 within the socket 68.

The piston 63 is controlled by the metering valve I which is operable to admit fluidunder pressure to the closed end of the cylinder 64 or to exhaustl fluid therefrom respectively upon an increase or decrease in the` pressure in the line I5, whereby the speed of the hydraulic motor 8 is maintained constant regardlessof varying load conditions. For example, if the load is increased, which results in increasedleakage, the motor 8 will tend to slow down and hence exhaust less fluid, but the resulting drop in the pressure built up by the orice restriction will immediately effect a compensatory increase in the pump de- Preferably, the valve l0 is built into the cover 20, and comprises a bore 1| opening therethrough 84 opening to the reduced intermediate section of inlet ports 82 .to the valve passage 1.1, and regis- .tering with a pressure supply passage 83 i the cover 20 and wall I8 connecting with the pump outlet passage 62. Formed in the endof ,the sleeve 16 at the opposite side of the groove 18 and longitudinally intersecting the valve passage 11 to denne exhaust ports are a plurality of bores the bore 1I. Two relief passages 85 and 86'open from the bore 1I at opposite ends of the sleeve 1.6 through the cover 2D to the interior of the pump housing I6 from where'spent fluid may-be discharged through a passage 81 to the pump inlet passage 59.

Reciprocallydisposed in the sleeve 16 is a valve member 88 ofthe spool type having two spaced peripheral grooves 89 and 99 opening respectively to the inlet and exhaust ports 82 and 84,' vand separated by a collar 9| adapted in intermediate position to block the control ports 19. To obtain accurate metering, the roots of the grooves 89 and 90 are tapered conically in opposite directions to restrict gradually the flow connection with the ports 19 as the valve member 88 is moved toward neutral position.

The valve member 88 is operable by a pressure responsive -element 92 and resilient pressure meansl 93 acting in opposition. In the present instance, the element 92 consists of a piston in abutment with one end of the valve member 88, and reciprocable in the pressure chamber 12. The control line i5 opens from the outer end of the chamber 12 to the direction valve 5 yfor connection with the exhaust line 9 when the motor lother end of the valve member 8B and seating against a collar 94 slidably guided in the intermediate section o the bore 1|. The collar 99 is integral with a pin 95, one end of which extends through the spring 93 into closely spaced relation to the valve member 88, and the other end of which is guided in a screw plug 96 closing the outer end of the spring chamber 13. The spring 93 is maintained under normal compression by a second spring 91 encircling the pin 95'and seating under compression at opposite ends against the collar 96 and the plug 96. The two springs 93 and 91 have different natural periods of fr'e- 16 is -xed in the bore 1I against the shoulder 14, l

and is formed with an axial valve passage 11. A peripheral groove 18 opens through radial ports 19 to the passage 11, and'is in registration with a passage B9 connecting with the closed end of the cylinder 64. The sleeve 16 is also formed at one side of the .groove 18 with a second peripheral groove 8l opening through a plurality of radial quency, so that any vibrations tending to result therein from pump pulsations are out of synchronism and neutralize or offset each other. Hence, the valve member 88 is maintained in sensitive balance without iluttering. A heavy coil compression shock spring 98 encircles the spring 91 and normally abuts at opposite ends against the shoulder 15 and the plug 96. The inner end convolution of the spring 98 projects inwardly for engagement by the collar 94 in the event of sudden and extended movement of the valve memyber 88 by the piston 92.

and` when in control is normally either closed' or slightly open'to permit only a restricted ow. The ow restriction in the valve I0 results in a large pressure drop in the passage .when con-A respectively to the motor lines Ii and 'I.

not reiiect or develop pulsations at critical motor speeds.

Circuit valves The direction valve 5 may be of any suitable character and for purposes of illustration is herein disclosed as comprising an outer casing 99 having an axial bore |00, and a reciprocatory valve member IOI of the spool type in the bore. Although the directionvalve 5 may be controlled either automatically or manually, the valve member IOI is shown connected to a hand lever |02. The delivery line 3 of the pump I is connected to a passage I 03 opening through ports |04 and |05 at spaced points to the bore |00. Intermediate these ports are two ports |06 and I0'I opening Intermediate these ports are two ports |08 and |09 'opening respectively to the exhaust line 9 and the control line I5. The valve member I has tWo peripheral grooves IIO and III dening spaced heads I|2 and II3 and II4. In one end position of the valve member I0| the groove IIO will connect the delivery line 3 to the motor line 6, and will connect the motor line I to the exhaust line 9. In the other end position these' connections will be reversed. In either end position, the valve member I0| will connect the control line I5 to the exhaust line 9. In the neutral or stop position, the heads I I2, I I3 and I I4 serve to block the-motor lines 6 and 'I from both the delivery line 3 and the exhaust line 9, and interrupt communication between the control line I and the exhaust line 9.

IOI is a passage II5, the opposite ends of Iwhich are movable respectively into communication with the inlet port |04 and the controlV port |09 upon movement of the direction valve 5 into in.- termediate'stop position, thereby connecting the delivery-side of the pump I directly to thercontrol line I5.

The selection valve I4 also may be of any suitable character and is herein shown as comprising a casing IIB and a valve member III adjustable by hand actuator I I8 to connect the exhaust line 9 to the sump 4 through any one of the three adjustable restricted orices I I, I2 and I3.

Operation cylinder B4 and causing the pump to deliver at l maximum capacity.

If the orifice restriction is now gradually increased, as by connecting the orifice I3 for fast feed, a pressure will build up in the control line I5, and act through the piston 92 to force the valve member 88 and pin 95| to the left until the collar 94 engages the shock spring 98 as shown in Fig. l. The valve member 8B will continue movement against the action of the spring 93 to connect the passages 80 and 83, whereupon the swash plate 45 will be adjusted to reduce the delivery of the pump to a point where the resultant pressure in the line I5 will maintain the valve I0 in closed position.

After the piston 63 is away from the stop plate Formed in the valve member I close the ports |26.

Y striction will be constant for all sizes of orifices,

and the volume of liquid flow therethrough will be constant for any one size but will vary with diierent sizes. Hence, the speed of the motor 8 for any given orifice setting will be constant regardless of varying load conditions and resulting variations in leakage in the pressure side of the system, and Will be determined'entirely by the Vsize of orice employed.A The delivery of the pump is fully utilized at all times in doing useful work, and is varied as required without the use of relief valves and other accessories commonly employed for by-passing excess uid.

The leveling adjusting of the swash plate 45 is gradual by reason of the metering characteristicsy of the valve I0. However, if the orice restriction should be changed or closed suddenly, a pressure impulse would be transmitted through the line I5 and cause the valve member 88 acting through the pin 95 to compress the shock spring 98, thereby permitting the pressure iluid to level oi the plate 45 immediately. Conversely, if there should be a sudden pressure drop in the line I5, the spring 91 would urge the valve member 88 far to the right to eiect an immediate and substantial increase in pump delivery. Upon movement of the reversing valve 5 into neutral position, the motor lines 5 and l' will be blocked, and the control line I5 will be disconnected from the exhaust line 9 and connected to the pump discharge line 3. Since the line 3 is alsootherwisc blocked, the delivery pressure of the pump will rise and effect movement of the swash plate 45 into level position to stop delivery.

`Modified pump control valve A modied form of metering valve II9 for c ontrolling the volume of pump delivery in response to the pump delivery pressure is illustrated in Fig. 8. The valve |I9 is built into the pump cover 20, and comprises a bore opening longitudinally therethrough land consisting of three sections |20, I2|, and |22. A valve sleeve |23 is fixed in the intermediate bore section |2I, and is formed with an axial passage |24, an external peripheral groove |25 opening vthrough radial ports |28 to the passage and communicating with the passage leading to the control cylinder 54, and a plurality of longitudinal bores I2'I and |28 in opposite ends longitudinally intersecting the valve passage |24 to dene inlet and outlet ports opening respectively to the bore sections |20 and |22. A passage |29 connects the bore section |22 to the interior of the casing IB. The bore section |20 is closed by a plug |30, and is connected to the pressure passage 83 leading from the pump outlet passage 62.

A valve member I3I isreciprocable in the passage I24, and is formed with spaced external peripheral grooves |32 and |33 defining an intermediate land I34 adapted in neutral position to The roots of 'the grooves |32 and |33 are conically tapered to eiect gradual valve restriction as the valve member I3I is moved toward closed position.

Fluid pressure in the bore section |20 tends -plus the subscript a.

to move the valve member |3| in a direction to connect the ports |26 and |2`| to effect a decrease 1n the pump delivery. A coiled compression .spring |35 opposes the pump delivery pressure,

and tends to move the valve member |3| in a direction to connect the ports |26 and E28 to effect an increase in the pump' delivery. The spring |35 is disposed in 'the bore section |22 and bears at one end against a shoe or retainer |36 fixed on the valve member |3|, and at the other end against an adjustable stop |31. Preferably, the stop |31 consists of a nonrotatable ,nut threadedon an adjustingscrew |38 and having longitudinal keys |39 slidable in slots |40 in a guide sleeve Ml. The sleeve mi is integral with a plug |62 threaded into and closing the outer end of the bore section |22. The screw |38 has a shoulder |43 disposed rotatably against the inner face of the plug |62, and has a shaft |44 extending axially through the latter for external adjustment. A. lock nut |45 on theshaft .|44 is available to clamp the screw |38 in adjusted position.'

The pump With the modified valve IIB is included as the source of pressure fluid in the hydraulic machine tool circuit shown in. Fig. l2. Since the pump is controlled directly from the delivery presure, this circuit does not include a control line, such as the line .|5, to the exhaust side of the system, but in all other respects it is like the circuit in Fig. '11, and hence like parts are identiecl by the same reference numerals In operation, when the motor torque is low and the largest orifice l lafis connected, the spring |35 will hold the valve member |3| to the right against the stop |30, so tha't the pump will deliver at maximum capacity. Upon an increase in pressure due to a load increase or a greater of pump delivery to said supply line in response to a predetermined pressure in said'circuit in-` fluenced by said delivery, whereby to maintain said predetermined pressure constant.

3. A hydraulic operating circuit comprising, in combination, a pressure supply line, an exhaust line, a, hydraulic motor adapted to receive fluidcombination, a pressure supply line, an exhaust line, a hydraulic motor adapted to receive iiuid from said supply line and to discharge spent uid to said exhaust line, adjustable ow restriction means in said-'exhaust line for building up a back pressure therein, and a variable delivery pump discharging to said pressure supply line, said pump including means for automatically controlling said pump delivery in response to said back presorice restriction, as by connecting the orifice |2'-" 40 or orifice ISH, the valve member |3| will be forced to the left against the action of the spring |35 to supply iiuid under pressure to the control cylin der 84. Thereafter, the valve H9 will maintain a constant maximum pressure, subject to adjustment by varying the initial tension of the spring |35, and will cause the pump to supply4 Whatever iiuid the system is able to take at this pressure.

We claim as our invention:

1. A hydraulic operating circuit comprising, in combination, a pressure supply line, a sump, an

exhaust line discharging to saidsump, a hydraulic 2. A hydraulic operating circuit comprising, in

combination a pressure supply line, an exhaust line, a hydraulic motor adapted to receive fluid from said supply line and to discharge spent fluid to said exhaust line, adjustable flow restriction means of predetermined xed area in said exhaust line for building up a back pressure therein, and a variable delivery pump discharging to sure to maintain said pressure constant under all load conditions and forall adjustments of said restriction means. g

5. A hydraulic operating circuit comprising, in combination, a pressure supply line, an exhaust line1 a reversible hydraulic motor having inlet and outlet lines, a direction valve for connecting said motor lines reversibly and respectively to' said supply and exhaust lines and interrupting said connections in,neutral position, a plurality of parallel restricted flow oriiices, selector valve means for connecting any one of said orifices in i said exhaust line to build up a back pressure therein, a variable delivery pump for delivering fluid under pressure vto said supply line, and control means for automatically adjusting said pump to vary the volume of delivery as required to maintain said back pressure constant during motor operation regardless of which orifice is connected in said exhaust line and to stop the delivery of saidpump upon movement of said valve into neutral position.

6. A hydraulicoperating circuit comprising, in combination, a, pressure supply line, an exhaust line; a hydraulic motor adapted to receive fluid from said supply line and to discharge spent fluid to said exhaust line, a plurality of ilow restriction orices, a valve for connecting said orices selectively in said exhaust line for building up a back pressure therein, and a variable delivery pump discharging to said pressure supply line, said pump including means for. automatically controlling said pump delivery in response to said back pressure to'maintain said pressure constant under all vload conditions and for all adjustments of said restriction means.

7. A hydraulic operating circuit comprising, in combination, a pressure supply line, an exhaust line, a hydraulic motor adapted to receive fluid from said supply line and to discharge spent iiuid to said exhaust line, flow restriction means in said exhaust line forbuilding up a back pressure therein, a variable delivery pump discharging t0 said pressure supply line, means for automatically adjusting said pump to control the pump delivery and including a control line, and a valve movable .into one position to disconnect said supply line said pressure supply line, said pump includingv means for automatically controlling the volume 4from said motor and to connect said control line to said supply line whereupon said means will stop pump delivery, and movable into another position to connect said supply line to said motor and said control line to said exhaust line Whereupon said means will maintain said back pressure constant.

8. A hydraulic operating circuit comprising, in combination, a pressure supply line, an exhaust line, a hydraulic motor adapted to receive fluid from said supply line and to discharge spent iiuid to said exhaust line,l Speed control ow restriction 10 means in said exhaust line for building up a. back pressure therein, a variable delivery pump discharging to said pressure supply line and including means for automatically controlling the pump delivery to maintain a constant maximum delivery pressure, and a start and stop valve in said supply line.

IRA J. SNADER. MAX A. MATHY S. 

